In the programmable controller (hereinafter, referred to as “PLC”) used for the factory automation (hereinafter, referred to as “FA”), the on/off information is input from an input device such as a switch or a sensor, and the logic operation is executed according to a sequence program (also referred to as the user program) written in the ladder language or the like. In accordance with the operation result obtained, the signal of on/off information is output to an output device such as a relay, a valve or an actuator thereby to perform the control operation.
The PLC is connected to the input device and the output device either directly or through a network. In the case where a network system connected by a network is constructed, the on/off information is transmitted or received through the particular network. In the process, the information is transmitted by the master-slave method in which the PLC normally acts a master unit and the device as a slave unit.
Recently, on the other hand, a fail-safe system has been introduced in the control operation with PLC. Specifically, the network as well as the PLC and each device are configured of a built-in safety function. The safety function is defined as a function to confirm safety and output data. The safety system is such that in the case where an emergency stop switch is depressed or a sensor such as a light curtain detects the intrusion of a person (a part of the human body) and the network system faces a dangerous situation, the fail-safe function works and the system turns to the safety side, thereby stopping the operation. In other words, the safety function described above allows a signal to be output only in the case where safety is secured in storage and to start the operation of a machine. In the case where no safety is assured, therefore, the machine stops.
The state of a given slave unit can be returned as a response by the slave unit that has received the request from PLC (master unit). As a result, PLC can acquire the status of the slave units connected to a network.
In the conventional network system, though the informed on the slave units connected can be recognized, the information on the input device or the output device connected to a slave unit cannot be recognized from PLC (master unit). The slave unit can be informed of the fact that a malfunction or a fault has occurred. In case of a malfunction, therefore, it is necessary to resort to the site of installation and specify the cause of the malfunction. In the case where a given part is required to be changed, an order for the part is issued and, with the newly acquired part, the installation site is visited again to change the part. This maintenance is complicated, and makes it difficult to take the necessary action quickly. It is still more difficult to monitor the life of the input and output devices connected to a slave unit from PLC, and therefore, a malfunction or fault is often handled only after it has actually occurred.
Even in the case where the installation site of a slave unit or a device connected with the slave unit is visited upon occurrence of a fault or the like, therefore, the status cannot be easily checked if the device is small or installed in the depth or behind other devices. As a result, the status of these input and output devices cannot be accurately grasped, and therefore the cause of a malfunction or fault may not be analyzed sufficiently.
Further, in the case of a network system having security means, the safety devices, though high in reliability and redundancy, are often shorter in life than the devices making up the conventional normal network. Once a malfunction of these safety devices occurs, it leads to the stoppage of the whole system, and therefore has a great disadvantageous effect on the whole system. Consequently, the function of monitoring the devices connected to a slave unit is more crucial than in the conventional system.
An object of this invention is to provide a slave unit and a network system as well as a slave unit processing method and a device information collecting method, in which the information on each device connected to a slave unit can be collected through a network.